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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
Geology and geochemistry of Pelagatos, Cerro del Agua, and Dos Cerros monogeneticvolcanoes in the Sierra Chichinautzin Volcanic Field, south of México City
Javier Agustín-Flores ⁎, Claus Siebe, Marie-Noëlle GuilbaudDepartamento de Vulcanología, Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán, 04510, México D.F., Mexico
a b s t r a c ta r t i c l e i n f o
Article history:Received 18 March 2010Accepted 9 August 2010Available online 19 September 2010
This study focuses on the geology and geochemistry of three closely-spaced monogenetic volcanoes that arelocated in the NE sector of the Sierra Chichinautzin Volcanic Field near México City. Pelagatos (3020 m.a.s.l.) isa small scoria cone (0.0017 km3) with lava flows (0.036 km3) that covered an area of 4.9 km2. Cerro del Aguascoria cone (3480 m.a.s.l., 0.028 km3) produced several lava flows (0.24 km3) covering an area of 17.6 km2.Dos Cerros is a lava shield which covers an area of 80.3 km2 and is crowned by two scoria cones: Tezpomayo(3080 m.a.s.l., 0.022 km3) and La Ninfa (3000 m.a.s.l., 0.032 km3). The eruptions of Cerro del Agua andPelagatos occurred between 2500 and 14,000 yr BP. The Dos Cerros eruption took place close to 14,000 yr BPas constrained by radiocarbon dating. Rocks from these three volcanoes are olivine-hypersthene normativebasaltic andesites and andesites with porphyritic, aphanitic, and glomeroporphyritic textures. Their mineralassemblages include olivine, clinopyroxene, and orthopyroxene phenocrysts (≤10 vol.%) embedded in atrachytic groundmass which consists mainly of plagioclase microlites and glass. Pelagatos rocks also presentquartz xenocrysts. Due to their high Cr and Ni contents, and high Mg#s, Pelagatos rocks are considered to bederived from primitive magmas, hence the importance of this volcano for understanding petrogeneticprocesses in this region. Major and trace element abundances and petrography of products from thesevolcanoes indicate a certain degree of crystal fractionation during ascent to the surface. However, the magmasthat formed the volcanoes evolved independently from each other and are not cogenetically related. REE,HFSE, LILE, and isotopic (Sr, Nd, and Pb) compositions point towards a heterogeneous mantle source that hasbeen metasomatized by aqueous/melt phases from the subducted Cocos slab. There is no clear evidence ofimportant crustal contributions in the compositions of Pelagatos and Cerro del Agua rocks. The Sr-isotopiccomposition of Dos Cerros, however, indicates a small degree of crustal contamination.
Pelagatos, Cerro del Agua, and Dos Cerros monogenetic volcanoesare located in the NE sector of the Sierra Chichinautzin Volcanic Field(SCVF) at the trenchward front of the Mexican Volcanic Belt (MVB)(Figs. 1 and 2). The SCVF comprises more than 200 monogeneticscoria cones and associated lava flowswhich are distributed in an areaof ca. 2500 km2 (Bloomfield, 1975;Martin del Pozzo, 1982, Siebe et al.,2004a). The total number of scoria cones in the SCVF is difficult todetermine due to the cover of older rocks by products of the youngesteruptions. Despite this, attempts to establish the cone density and thetotal erupted volume have been carried out. Bloomfield (1975)reported a scoria cone density of 0.1/km2. Swinamer (1989) estimateda total volume of erupted products of 1600 km3. By contrast, Márquez
et al. (1999b) calculated a total volume of 470 km3, which is morethan twice the modest estimate of ca. 200 km3 reported by Siebe et al.(2004b). Siebe et al. (2005) calculated an effusion rate of 0.8 km3/1000 yr for the entire field.
Paleomagnetic data suggest that the SCVF volcanism is younger than0.7–0.8 My (Mooser et al., 1974; Herrero and Pal, 1978; Urrutia-Fucugauchi and Martin del Pozzo, 1991). In addition, morphologicalobservations and radiocarbon dating indicate that a great number ofscoria cones and lava flows are younger than 40,000 yr BP (Bloomfield,1975; Martin del Pozzo, 1989; Kirianov et al., 1990; Siebe, 2000; Siebeet al., 2004b; Siebe et al., 2005). The majority of volcanic rocks in theSCVF are basaltic andesites, andesites, and dacites of the calc-alkalinesuite (Bloomfield, 1975; Wallace and Carmichael, 1999), but somealkaline and calc-alkaline basalts have also been reported (Márquezet al., 1999b; Wallace and Carmichael, 1999; Siebe et al., 2004b).
In recent years, the SCVF has become an important subject of studybecause of its close proximity toMéxico City (Fig. 2) and its hazard andrisk implications for a considerably large population. The frequency ofLate-Pleistocene/Holocene eruptions in the SCVF (ca. 1/1200 yr)(Siebe et al., 2005) and the youthfulness of its most recent eruptions
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⁎ Corresponding author. Departamento de Vulcanología, Instituto de Geofísica,Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, 04510,México D.F., Mexico. Tel.: +52 55 56224119; fax: +52 55 55502486.
(e.g., Xitle volcano, 1665±35 yr BP, Siebe, 2000; Chichinautzinvolcano, 1835±55 yr BP, Siebe et al., 2004a) underscore theimportance of these studies.
The purpose of this paper is to present results derived fromdetailed geological mapping in the NE sector of the SCVF, whichallowed the estimation of the volumes of products (scoria cones andlava flows) emitted by Pelagatos, Cerro del Agua, and Dos Cerrosmonogenetic volcanoes. A stratigraphic and petrographic descriptionof erupted rocks is also provided. The ages of the volcanoes wereconstrained with the help of the “Tutti Frutti” pumice stratigraphicmarker, a Plinian fallout deposit from Popocatépetl volcano eruptedca. 14,000 yr BP (Siebe et al., 1995, 1999; Siebe and Macías, 2006). Inaddition, the origin of the magmas that produced these volcanoes isdiscussed on the basis of newmineralogical, geochemical, and Sr–Nd–Pb isotopic data.
Pelagatos volcano stands out among other studied monogeneticvolcanoes in the SCVF by its relatively small erupted volume (~0.031–0.035 km3 DRE) and the primitive nature of its rocks (high Cr, Ni, andMgO contents and high #Mg) (Guilbaud et al., 2009). For thesereasons, Pelagatos is a good indicator of petrogenetic processesoccurring at mantle depths underneath the SCVF. In summary, thisstudy contributes to the understanding of monogenetic volcanism byidentifying subtle differences in the generation and evolution ofascendingmagma batches, their eruptive styles, and the emplacement
of products of three young and closely-spaced volcanoes locatedwithin the same volcanic field.
2. Regional geology and tectonic setting
The MVB is an E–W oriented volcanic arc which runs from thePacific Ocean to the Gulf of México along a stretch of ca. 1000 km(Fig. 2). The SCVF is located at the front of the central MVB to the S ofthe lacustrine basin of México City, which is situated at ~2240 m a.s.l.Before the Pleistocene, the basin drained to the S (Fries, 1960), but thesouthern drainage was blocked by formation of the SCVF (Mooser,1963). The SCVF is an E–Welongated volcanic range that rises to morethan 3500 m a.s.l.
Volcanism in the central and eastern part of the MVB is associatedwith the subduction of the Cocos Plate beneath the North AmericanPlate (Pardo and Suárez, 1995; Ferrari et al., 1999, Gómez-Tuena et al.,2007a) (Fig. 2). Pérez-Campos et al. (2008) estimated a verticaldistance of 150 km from the Earth´s surface to the subducting slabbeneath the Mexico City area where it abruptly plunges with a dipof ~75° into the mantle after initially having a horizontal configura-tion. The onset of MVB volcanism around the basin of México datesback to the Early Miocene (Gómez-Tuena et al., 2007a) and theTepoztlán formation belongs to this early volcanism (García-Palomoet al., 2002; Ferrari et al., 2003; Lenhardt et al., 2010). It consists of a
Fig. 1. Sketch map of the SCVF and adjacent populated areas and landmarks (modified from Siebe et al., 2004a). Pelagatos, Cerro del Agua, and Dos Cerros are outlined in a rectanglewhich is shown in detail in Fig. 3.
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ca. 1000-m-thick sequence of pyroclastic, lahar, and lacustrinedeposits (García-Palomo et al., 2002; Lenhardt et al., 2010) thatunderlies the SCVF and crops out at its southern margin (Siebe et al.,2004a; Lenhardt et al., 2010). The Pliocene Sierra de las Cruces(Fig. 1) is located to the NW of the SCVF (Fig. 1) (Delgado-Granadosand Martin del Pozzo, 1993; Osete et al., 2000; Arce et al., 2008) andconsists of a succession of overlapping volcanic edifices, lava flows,lahars, and pyroclastic deposits (Mooser et al., 1974, Osete et al.,2000). This activity migrated towards the SE and produced theAjusco Formation which consists of several andesite lava domes ofMiddle Pleistocene age (Osete et al., 2000) (Fig. 1). The Mexico Citybasin is bounded to the E by the N–S oriented Sierra Nevada (Fig. 1)which is a Plio-Pleistocene to recent volcanic range (Nixon, 1989).From N to S, it consists of the andesitic–dacitic Tláloc, Telapón,Iztaccíhuatl, and Popocatépetl strato-volcanoes.
Beneath the volcanic deposits lies a thick sequence (N2000 m) ofCretaceous marine dolomites and clastic limestones that was foldedduring the Laramide orogenesis (Fries, 1960, 1962). The sequenceincludes the Mexcala, Cuautla, Morelos, and Xochicalco Formationsand is located at ~2000 m below the ground level of the basin ofMéxico (Mooser, 1970), although parts of it do crop out in the Valleyof Cuernavaca to the S (Mooser, 1970). The composition and age of thebasement below the Cretaceous sequence is poorly known. During theEarly and Middle Tertiary, the Balsas Group, a sequence of redbeds,gypsum, lacustrine, and volcaniclastic sediments was deposited(Fries, 1960, 1962.
According to Ferrari et al. (1994) the Plio-Quaternary volcanism ofthe MVB is related to trans-tensional and extensional faulting.Although faults are difficult to recognize in the SCVF, few E–W-trending, northward-dipping faults have been identified (Siebe et al.,2004a). Furthermore, in the surroundings of the basin of México it hasbeen possible to distinguish the occurrence of N–S oriented chains ofpolygenetic volcanoes (e.g., the Sierra Nevada mentioned above) androughly E–W aligned monogenetic centers (e.g. in the SCVF: Mooseret al., 1974; Bloomfield, 1975; Alaniz-Álvarez et al., 1998; Siebe et al.,2004a). These alignments of monogenetic centers are related to a N–Sextensional regime (Márquez et al., 1999b) whose associated faultsserved as pathways for small magma batches that eventually erupted
and formed monogenetic volcanoes (Siebe et al., 2004a). In addition,Siebe et al. (2004a) postulated (based on satellite imagery) that theSCVF is an E–W-trending horst whose northern part is step-faultedand accommodated most of the vertical and horizontal movementthat occurred between the block of Morelos in the S and the subsidedblock represented by the basin of México to the N. Shallowearthquakes (b20 km) of small magnitude (b4 on the Richter scale)recorded in the SCVF during the past decades indicate that the area istectonically active (Márquez et al., 1999b and references therein).
3. Morphology, estimated volumes, and stratigraphy
3.1. Methods
Morphometric parameters of scoria cones have been reported inprevious volcanological studies in the SCVF. Apart from merelydescribing the morphology of volcanoes, several authors have alsoinferred the eruptive styles and relative ages of scoria cones and theirassociated lava flows (Bloomfield, 1975; Martin del Pozzo, 1982,1989; Swinamer, 1989; Rodríguez-Lara, 1997; Arana-Salinas, 2004;Siebe et al., 2004a, 2004b, 2005; Guilbaud et al., 2009). Márquez et al.(1999b) estimated the morphometric parameters of 181 cones fromthe SCVF: the mean cone height (Hco), cone basal diameter (Wco),and crater diameter (Wcr) are 104, 644, and 225 m respectively. TheHco/Wco ratios vary from 0.004 to 0.525. These values are similar tothose reported by Hasenaka and Carmichael (1985) for theMichoacán–Guanajuato Volcanic Field. The above relations werefirst established elsewhere by Porter (1972) and reviewed by Wood(1980b). Wood (1980b) concluded that the Hco/Wco relation andthe slope angle of cones decrease mainly due to degradation byerosion, which in turn is mostly controlled by climate. Therefore, theage of a scoria cone may be inferred by determining its morpho-metric parameters (Porter, 1972; Damon et al., 1974; Moore et al.,1976; Wood, 1980a, 1980b; Hooper and Sheridan, 1998). However,the previous statement should be taken with care as theseparameters vary from field to field and even from volcano tovolcano within the same field due to differences in climate regimesand eruption styles (Wood, 1980a). In this study, the morphometric
Fig. 2. Map showing the distribution of volcanism in the Mexican Volcanic Belt (MVB) since the Neogene (modified from Ferrari et al., 2000). The Plio-Quaternary MVB appears inlight pink. The iso-depth contours of the subducting plate (Pardo and Suárez, 1995) are shown in dashed and solid lines, the dashed lines indicate extrapolated countours. The SierraChichinautzin Volcanic Field (SCVF) is at the trenchward front of the MVB.
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parameters of the scoria cones and the areas and volumes of lavaflows were calculated by using different tools: a 1:50,000 topo-graphic map, the Google Earth program (available on internet) anddirect altitude measurements in the field with an altimeter, and/or aGPS instrument. The areas covered by the lava flows were obtainedwith the aid of a digital planimeter together with the topographicmap. The lava volumes were calculated by multiplying the lava flowarea with its average thickness. The volume of the scoria cones wasestimated as the average of the results obtained by applying theformula of a truncated cone and the method proposed by Riedelet al. (2003) (the volume occupied by vesicles or interstices was notconsidered). Note that despite the uneven morphology of thePelagatos and Cerro del Agua cones, and thus the difficulties incalculating their volume (see below), the morphometric parametersof these cones (Table 1) fall well within the ranges given byMárquez et al. (1999b).
The SCVF is composed of overlapping Quaternary sequences ofscoria cones, lava flows, ash fallout deposits, reworked material, andsoils. Some authors have described the stratigraphy of thesesequences (Mooser et al., 1974; Bloomfield, 1975; Martin del Pozzo,1982; Márquez et al., 1999b; Wallace and Carmichael, 1999; Siebeet al., 2004a, 2004b, 2005; Guilbaud et al., 2009). However, so far it hasnot been possible to establish a complete chrono-stratigraphicsequence that includes the majority of the volcaniclastic depositsand lava flows comprising the SCVF. One reason for this is the lack ofan extensive age data set (Ar–Ar and 14C). Nonetheless, two importantstratigraphic markers have been identified and used to determine arelative age of the sequences: the “Tutti Frutti” pumice fallout depositwith an age of 14,000 yr BP (Siebe et al., 1995, 1999; Siebe andMacías,2006) and the “Upper Toluca” pumice fallout deposit with an age of10,500 yr BP (Macías et al., 1997; Arce et al., 2003). The formeroriginated from the andesitic Popocatépetl and the latter from thedacitic Nevado de Toluca volcano. The area occupied by the Pelagatos,Cerro del Agua, and Dos Cerros volcanoes is well within the area ofdeposition of the “Tutti Frutti” deposit. Its occurrence in the study areawas crucial for establishing relative ages for the three eruptions.
3.2. Results
The scoria cones and their associated lava, scoria, and ash productsare shown on Figs. 3, 4, and 5. Morphometric parameters and eruptedvolumes are listed in Table 1. The stratigraphic relationships betweenproducts from individual eruptions and the underlying deposits aredepicted on idealized columns shown in Fig. 6.
3.2.1. Dos CerrosDos Cerros is a lava shield crowned by two scoria cones aligned in
an E–Wdirection: the Tezpomayo scoria cone (3080 m a.s.l.) to theW,and the La Ninfa scoria cone (3000 m a.s.l.) to the E (Figs. 3 and 4a,b).On morphological and stratigraphical grounds, the lava shield and thetwo cones are considered the products of the same monogeneticeruption. Older scoria cones, such as Ayaqueme and Cuajomac (Fig. 3)were partially covered by the flows.
The N–S elongated Tezpomayo cone displays two breaches, asmaller on its NE and a larger on its SW flank (Fig. 4b). The effusionof a lava flow (F3 in Figs. 3 and 4a,b) that emanated from the SWbreach during the late stages of the eruption (the flow is not coveredby ash) may have been the cause of this breaching event. Lavaeffusion may have occurred concurently with some mild explosiveactivity at the vent. These explosions may have been poor in ash(McGetchin et al., 1974) or the ash produced may have been carriedby the wind towards the east, hence not covering the flows (seebelow for wind direction evidence). La Ninfa cone is also elongatedin the N–S direction, but has a wider diameter (Fig. 4b). Its slopesare scarcely vegetated. Both cones are well preserved and theirflanks slope at a ~30° angle. Notably, the eastern cone rims ofTezpomayo and La Ninfa are the highest and concentrate the largestvolume of accumulated pyroclastic material (Fig. 4b). This might bethe result of predominant wind direction at the time of the eruption.The fact that both cones were affected by similar weather conditionsis consistent with coeval formation. Tezpomayo and La Ninfamorphometric parameters (Table 1) fall within the average forcones in the SCVF and other volcanic fields in the world (Márquezet al., 1999b; Schmincke, 2004). Their Hco/Wco relations of 0.16 areclose to the 0.18 value for fresh cones (Porter, 1972) and pointtowards a recent age.
The Dos Cerros lava shield is constituted by numerous overlappingblocky lava flows that are difficult to distinguish individually, exceptfrom a late lava flow (F3). This flow first surrounded Tezpomayo conebefore going around La Ninfa cone (Figs. 3 and 4a,b). Apart from thisflow that is thickly forested (Fig. 4a), the lava shield is scarcelycovered by pine and oak trees and shrubs that grow on a moderatelydeveloped soil.
Lava thicknesses ranging from 5 to 30 m were measured in thefield. However, as the shield was emplaced on older lava flows and ona relatively steep slope (~7°) it is difficult to estimate the averagethickness of lava flows. A 15 m average thickness for calculating lavavolumes was used (Table 1). The lava volume (1.2 km3) and the areacovered by the shield (80.3 km2) are among the largest in the entireSCVF (Table 2).
Table 1Morphometric parameters for Pelagatos, Cerro del Agua, and Dos Cerros monogenetic volcanoes.
Although an emission vent is not discernible, the E–W alignedcones and also the E–W elongated shield point toward an initialeffusive phase from an E–W fissure. Valentine and Gregg (2008) statethat many (if not most) continental mafic eruptions begin as fissureeruptions. In addition, Strombolian eruptions may start withHawaiian-style lava fountains (Vergniolle and Mangan, 2000). Earlylava flows were distributed to the north-northwest of the continentaldrainage divide and to the east, towards the valley of Amecameca(Fig. 3). They were emplaced roughly radially from the vent and form
a lava shield. Monogenetic structures may form by many eruptiveepisodes; as a result, a compound flow field (as Dos Cerros) isemplaced consisting of numerous lava flows that interfinger,anastamose, and pile up on top of each other in a complicatedmanner (Valentine and Gregg, 2008).
Based on the evidence for a late flow (see above), it is believed thatthe Dos Cerros eruption ended in an effusive manner. Tezpomayo andLa Ninfa scoria cones represent the Strombolian phase of the eruptionthat occurred sometime between the initial and final effusive activity,
Fig. 3. Schematic geologic map of the NE sector of the SCVF where Pelagatos, Cerro del Agua, and Dos Cerros monogenetic volcanoes are located. Sample sites are also shown.
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and may have been contemporaneous with lava emission. The SWflank of La Ninfa cone is quarried (Fig. 4b), which allows observationof the scoria layers that compose the cone. In the La Ninfa cone quarry,the lapilli-sized scoria clasts are sub-angular and form layers that
range from a few cm to ~50 cm in thickness. The scoria beds aremainly clast-supported (Fig. 5a) and exhibit crude reverse grading.The presence of coarse-to-medium ash grains between scoriafragments is common. Bed grading is less evident towards the
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inner-cone facies, but sorting improves. The scoria layers are roughlylaterally continuous and rest at angles that range from 20° to 40°. Theyare cut by sub-vertical fractures that dip towards the outer cone facies(i.e. towards the west), and sometimes cut across the entire thicknessof the deposit, producing marked offsets in the layers. These are likelyproducts of late gravitational instabilities in the cone flanks. The scoriashow alteration colors along the fractures and display grey andredish–yellowish patches due to post-depositional gas alteration. Inaddition, scoria layers within the cone have distinct colorations whichare laterally continuous and thus not linked to post-eruptivealteration. Interbedded and fractured “spatter” bombs are also presentthroughout the quarry. Vesicularity ranges from moderate in scoriaeto scarce in bombs and vesicles are usually subrounded to slightlyelongated with an average size of 3 mm. The abundance of denseangular blocks within the deposit is notable. A loose to partly-indurated deposit of ash layers between 1 and 2 m-thick (Fig. 5a)overlies the lapilli beds in the outer-wall facies of the La Ninfa cone.The layers consist of medium-to-coarse sized ash fragments ofscoriaceous glass shards and broken crystals of olivine, pyroxene,and plagioclase. These ash deposits are locally partly reworked. Theseobservations will be discussed further in Section 3.2.3.
The uppermost parts of the outer cone talus are composed oflocally-reworked material, which is overlain in some parts by the“Tutti Frutti” pumice fallout deposit that consists of brown-orangepumice clasts and abundant granodiorite, limestone, and skarnxenoliths. Interestingly, to the SE the “Tutti Frutti” pumice overliesdirectly ash deposits from the Dos Cerros eruption (site 0529, Fig. 5b).The lack of a paleosol and layers of reworked material between thesedeposits indicate that their emplacement occurred around the sametime close to 14,000 yr BP (Fig. 6a). This is confirmed by a radiocarbonage of 13,980±70 yr BP obtained from charcoal found in an ashdeposit from Dos Cerros (data provided by Lilia Arana–Salinas fromsample TML-65, Lab.# AA-50118, 19°08′20″N, 98°58′48″W, 3000 m a.s.l.). The quarry of Tezpomayo cone also shows the “Tutti Frutti”pumice lying directly on top of the uppermost part of the cone's scoriaand ash deposits. Hence, the two Dos Cerros cones formed shortlybefore the cataclysmic “Tutti Frutti” eruption from Popocatépetl.
3.2.2. Cerro del AguaCerro del Agua monogenetic volcano consists of a truncated cone
and associated blocky lava flows (Figs. 3 and 4c,d). The cone(3480 m a.s.l.), called Nepanapa by the local population, has a
Fig. 5. a) Outer wall of La Ninfa scoria cone deposits. The topmost layers of the sequence consist of indurated ash. Note the clast size gradation ranging from lapilli to coarse-ash.Shovel for scale is ca. 1 m (photograph by M.-N. Guilbaud, Nov. 16, 2007). b) Dos Cerros ash fallout deposits overlain by the “Tutti Frutti” Plinian pumice stratigraphic marker (site0529 in Fig. 3). Note the sharp contact between the two deposits. Scale is ca. 20 cm long (photograph by C. Siebe, Aug. 18, 2005). c) Pelagatos scoria cone deposits exposed on its SWquarried flank (photograph by M.-N. Guilbaud, Feb. 14, 2007). Instability of quarry walls due to lose scoria aggregates are evidenced by the thick talus apron at the base of the wall.Bedding can be recognized in the upper right side of the picture. d) Outer-cone wall of Pelagatos scoria deposits (photograph byM.-N. Guilbaud, June 6, 2007). Note the coarse size ofclasts and a thin coarse-ash layer at the base of the beds. The fractured bomb encircled in a red line is ca. 1 m long. The continuity of beds is visible in this part of the deposit.
Fig. 4. a) Aerial view of Dos Cerros scoria cones from the SE (photograph by Claus Siebe, Dec. 19, 1994). Tezpomayo and La Ninfa scoria cones are in the foreground. The last lavaflow to be emplaced (F3) is thickly forested and partially delineated with a broken line. Arrows indicate lava flow directions. b) Satellite image (modified from Google Earth) ofTezpomayo and La Ninfa scoria cones. Tezpomayo cone is breached on its SW flank. The broken line encircles the late lava flows (F3) and arrows indicate that they originated atTezpomayo crater. Note the E–W alignment of the two cones. c) Aerial view of Cerro del Agua from the ENE (photograph by Claus Siebe, Oct. 29, 1996). Cerro del Agua cone andassociated lava flows are outlined with solid contours. Cerro del Agua products were emplaced on top of the older Cilcuayo lavas. Arrows indicate lava flow direction. d)Satellite image (modified from Google Earth) of Cerro del Agua scoria cone. The crater rim is opened to the SE and outlined with a solid line. Arrows indicate lava flow direction.e) Aerial view of Pelagatos from the E (photograph by Claus Siebe, Dec. 29, 1994). Pelagatos cone and associated lava flows are highlighted with solid contours. Arrows indicateflow direction. Few eroded older scoria cones are visible in the foreground. f) Satellite image (modified from Google Earth) of Pelagatos scoria cone. The crater rim is outlinedwith a solid line. The cone is open to the SSE and has a quarry on its SW flank. Arrow indicates lava flow direction.
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horseshoe-shaped crater formed by a collapse to the SE (Fig. 4d).Similarly to the Pelagatos cone (see below), the Cerro del Agua cone(with an average slope angle of 28°) was emplaced on the Cilcuayolava field, but on a steeper slope, a situation that contributed to flankfailure. As in the case of Pelagatos (see below), estimating themorphometric parameters posed a problem because of the highlyirregular shape of the cone. Hence, it was necessary to use the craterrim and flank remains to approximate the original dimensions of thecone. The dimensions of this cone fall within the average for scoriacones in the SCVF referred by Márquez et al. (1999b). The Hco/Wcorelation is 0.13 (Márquez reports a value of 0.118). This value departsfrom the 0.18 value for fresh scoria cones (Porter, 1972), but is closerto the value of 0.125 for cones of 0.2–0.9 Ma in age as defined byColton (1967). Certainly, the morphology of this volcano does notcorrespond to the age assigned to the cones studied by Colton, and theincongruous value is probably the result of the uneven cone shape.Scoria and ash beds are poorly exposed due to cover by vegetation andlack of outcrops. Hence, it is not possible to provide a description oftheir volcaniclastic characteristics.
The volume and area covered by the lava flows is shown in Table 1.The vent source of the lava flows is located to the S of the continentaldrainage divide. Initially, the lava flowed to the SE, and then to the Stowards the valley of Cuautla (Fig. 3). The proximal parts of the lavaflows were emplaced on the Cilcuayo lava field (Figs. 3 and 4c).Although it was possible to measure up to 30 m-thick proximal lavaflows, it was difficult to estimate their average thicknessmainly due to
the unknown pre-existing topography. A mean average thickness of15 m was considered to calculate the lava volume (Table 1).
The lava flows are blocky and it is possible to observe pinkish-to-reddish colored basal breccias at distal outcrops. A dense pine forestgrows on a moderately developed soil on top of the lava flows.Although reworked layers of the “Tutti Frutti” pumice were foundoverlying the Cilcuayo products in this area, Cerro del Agua productsare not covered by this stratigraphic marker. Hence, the Cerro delAgua eruption must be younger than 14,000 yr BP (Fig. 6b).Unfortunately, we were not able to find charcoal to determine aradiometric age for Cerro del Agua.
3.2.3. PelagatosPelagatos monogenetic volcano consists of a main scoria cone, two
secondary scoria ridges, and associated blocky lava flows overgrownby a pine forest (Figs. 3 and 4e,f).
The main Pelagatos cone (3020 m a.s.l.) has a horseshoe-shapedcrater opened to the SE (Fig. 4f). The fact that the cone rests on theinclined Cilcuayo lava field (Figs. 3 and 4e) may have contributed tocone instability and subsequent collapse of its S flank (see alsoGuilbaud et al., 2009). Gutmann (1979) observed in the Pinacatevolcanic field (Sonora, México) that many of the cones that wereemplaced on sloping ground had craters opened towards the down-slope side of the cone. Moreover, Valentine et al. (2006) suggestedthat when a cone is emplaced upon a sloping surface, the downhillflank of the cone will be in a state of tension. A late lava flow (F2 in
Table 2Estimated volumes of recently mapped single volcanoes in the SCVF. Note that total volumes vary greatly (by two orders of magnitude) and only few volcanoes produced N1 km3 oflava. Radiocarbon ages and SiO2 compositions are also provided. Data from Siebe et al. 2004a, 2004b(1); Arana-Salinas, 2004, Siebe et al., 2005(2); and this study(3).
Fig. 6. Idealized stratigraphic columns for a) Dos Cerros, b) Cerro del Agua, and c) Pelagatos monogenetic volcanoes. Description of cone deposits of Dos Cerros and Pelagatos arebased on exposures at quarries.
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Figs. 3 and 4e) appears to have emanated from the base of thePelagatos cone, additionally promoting flank collapse as reported forthe case of Paricutin volcano (Foshag and González, 1956). It ishowever not possible to determine whether the cone was brokenfrom inside, as in the case of lateral pressure exerted by a lava pond, orfrom below when the lava flow was fed by a dike beneath the cone ora bocca at its base (see discussions in Valentine and Gregg, 2008).
The early lavas flowed to the S, but soon became diverted to the NEtowards the Juchitepec valley (Fig. 3) moving down on a ~2.8° slope.The lava flow thickness varies from 5 to 10 m and it is possible todistinguish at least two main lava flows. Flow 1 (F1 in Figs. 3 and 4e),with a thickness of ~5 m, was emplaced first and extends out for ca.7 km. The second flow (F2 in Figs. 3 and 4e), with a thickness of~10 m, has a length of ca. 1.5 km. It is difficult to determine anaccurate average thickness because the pre-existing topography isunknown. An average thickness of 7.5 mwas assumed for lava volumecalculations.
The two scoria ridges formed during the eruption are aligned on anENE–WSW direction with respect to the main cone. The horseshoe-shaped ridges are open to the S and not more than 30 m high.Guilbaud et al. (2009) interpreted these mounds as the remains of aninitial episode of activity. Their volume is minor in comparison to thatof the main cone, therefore it was not taken into account for volumeestimates.
The morphometric parameters and emitted volumes of Pelagatosare listed in Table 1. Because of the irregular shape of the cone, it wasnecessary to reconstruct its original shape (i.e. prior to collapse) inorder to compute its volume. This was done by extrapolating from theremains of the crater rim and the flanks. The small dimensions of thePelagatos cone and lava flows are evident. The Hco/Wco ratio is 0.16,whereas Márquez et al. (1999b) and Guilbaud et al. (2009) reportvalues of 0.24 and a range of 0.14 to 0.17 respectively. These smalldifferences are the consequence of dealing with an irregular cone.Nevertheless, the Hco/Wco relation approximates the value of 0.18 forfresh cones found by Porter (1972). The cone volume of 0.0017 km3,on the other hand, is in agreement with the volume estimatedpreviously byMárquez et al. (1999b). The cone flanks have an averageslope angle of 29°.
The SW flank of the Pelagatos cone has been quarried exposingproximal fallout deposits (Figs. 4f and 5c). A detailed description ofthese is provided in Guilbaud et al. (2009). In summary, the depositsare clast-supported and composedmainly of medium-to-coarse lapillito bomb-sized, dark gray to bluish scoriae (Fig. 5d). Ash-sizedparticles are scarce, but thin coarse-ash layers are exhibited at thebase of reversely graded layers in the outer-wall deposits. Scoria clastsin the uppermost layers show a yellowish color due to weatheringalteration. Otherwise, they frequently present an iridescent sheen.Olivine crystals and quartz xenocrysts in the scoriae are visible to thenaked eye. Vesicularity varies from high in themost abundant angularscoria fragments, to moderate/poor in the scarse dense fragments.Vesicles are subrounded to elongated in the dense fragments, andround in the vesicular clasts. Deposits consist of roughly continuous tolenticular beds with an average thickness of 50 cm. They rest at anglesthat vary from 20° to 30° and are poorly to well sorted.
The size of fragments (coarse lapilli to bombs) and the paucity ofash-sized fragmentsmay suggest that the Pelagatos cone building wasthe product of a low-energy Strombolian activity (see discussion inValentine and Gregg, 2008). In contrast, at Dos Cerros, the abundantash, low to moderate scoria vesicularity, and overall smaller clast size(see above) point to a more energetic Strombolian eruption. On theother hand, the high vesicularity and the presence of quartzxenocrysts in the Pelagatos scoria indicate an important volatilephase at the time of eruption and rapid magma ascent (no time forassimilation after entrainment) respectively (Guilbaud et al., 2009).Also, the angularity and high vesicularity of the majority of the clastsin the Pelagatos cone imply high ejection heights (Guilbaud et al.,
2009). In contrast, at Dos Cerros, the abundance of dense clasts in thequarried La Ninfa cone might have resulted from important clastrecycling at the vent, which could also have caused the abundance offine-grained particles and the high degree of oxidation of the clasts(coloration of the layers). This would imply low ejection heights.
A charcoal sample was collected at site 0532 (Fig. 3) from a silty-to-clayey ochre soil. This soil contains admixed lava fragments andleans against Pelagatos lavamargins indicating that it formed after thePelagatos eruption. Thus, since a radiocarbon age of 2520±105 yr BPwas obtained, this dating should be considered as a minimum age forPelagatos. Because the “Tutti Frutti” deposit was not found coveringthe Pelagatos products, a 14,000 yr BP age can be regarded as amaximum age. This means that the Pelagatos eruption occurredduring the time interval between 14,000–2520 yr BP (Fig. 6c). Basedon the same reasoning, this age interval is also valid for the Cerro delAgua eruption. Undoubtedly, this large time interval is not accurateenough for making eruption recurrence calculations for the entirevolcanic field, but is sufficient to corroborate the Late-Pleistocene–Holocene age of both Pelagatos and Cerro del Agua volcanoes.
4. Mineralogy and petrography
Petrographic characteristics of SCVF rocks have been describedpreviously (e.g., Martin del Pozzo, 1989; Swinamer, 1989; Rodríguez-Lara, 1997; Márquez et al., 1999b; Wallace and Carmichael, 1999;Verma, 2000; Velasco-Tapia and Verma, 2001; Arana-Salinas, 2004;Siebe et al., 2004a, 2004b, 2005; Guilbaud et al., 2009). In general,SCVF rocks are aphanitic and seriate porphyritic (b25 vol. %). Olivine(Ol) and clinopyroxene (Cpx) are ubiquitous phenocryst phases inmost SCVF basalts and basaltic andesites, but orthopyroxene (Opx)and plagioclase (Plg) phenocrysts also occur. Hornblende (Hbl) andbiotite (Bt) are uncommon in SCVF rocks. The groundmass of SCVFrocks is composed of the same phenocryst phases in addition to Fe–Tioxides and scarce zircon and apatite in the most silica-rich products.
Thin sections were prepared and observed under the polarizingmicroscope. The modal composition of selected Pelagatos, Cerro delAgua, and Dos Cerros samples is shown in Table 3. The composition ofcrystals was deduced from their optical characteristics, except forPelagatos for which compositional data is available (Guilbaud et al.,2009). Pelagatos, Cerro del Agua, and Dos Cerros rocks are aphaniticand seriate porphyritic (≤10 vol.%, Table 3) basaltic andesites andandesites. Phenocrysts (crystals N0.25 mm) have an average size of1 mm and typically occur in the following mineral assemblage:OlNCpxNOpxNPlg (note that Pelagatos samples only contain Olphenocrysts, see below). Spinel (Sp) inclusions are common in Olphenocrysts. The hyalo-ophitic groundmass consists of abundant Plg-laths, which occasionally display a trachytic fluidal texture withmicrocrystals of Ol, Cpx, Opx, and oxides (mostly magnetite, Mt).
Pelagatos rocks exhibit only Ol as the phenocryst phase (between8.0 and 10.5 vol.%, Table 3). These phenocrysts are embedded in aglassy groundmass composed of Plg-laths, oxides, and Ol, Cpx, andOpx microcrysts. The subhedral to euhedral Ol phenocrysts displayeuhedral chrome-spinel inclusions, which have regular polygonalshapes and are dark, amber, or brownish in color. Some of thephenocrysts show reaction bays as an evidence of disequilibrium.Guilbaud et al. (2009) report that the Ol phenocrysts display Mg-enriched cores (Fo87–91) compared to the less Mg-enriched rims(Fo83–87). They occur as separate crystals or in clusters up to 3 mm indiameter. Cpx (augite) occurs only in the groundmass and does notdisplay pronounced disequilibrium textures. Some Cpx microcrystsare intergrown with Plg. On the other hand, Plg-laths are thedominant phase in the groundmass (Table 3) and are arranged in afluidal texture. They have a labradorite composition (An48–An69)(Guilbaud et al., 2009). Opaque oxides are present in the groundmassas subhedral and euhedral crystals. Quartz xenocrysts are rare, but
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were observed in Pelagatos products: some of them are rimmed byaugite coronas and some are not.
The abundance of oxides in the groundmass is high in Pelagatos,Cerro del Agua, and Dos Cerros (Table 3) and could be related, at least inthe case of Pelagatos, to a high initial dissolved H2O-content (b5 wt.%,Guilbaud et al., 2009). In addition, the pyroclastic products of Pelagatoscontain lesser amounts of oxides than earlier-emplaced lavas whichmight be the consequence of a change in the eruptive style due inpart tooxidation processes operating in the early stages of the eruption(Genareau et al., 2010).
Cerro del Agua rocks are slightly more silicic than Pelagatos rocks.Ol±augite phenocrysts occur as separate crystals (0.5–1.0 mm) or asglomero-phenocrysts (from 1 to 3 mm) associated to minor Plg andOpx. Both Ol and Cpx show spinel-inclusions as described above forPelagatos. Cpx does not show important disequilibrium textures, butOl displays reaction bays and skeletal forms. Opx and Plg phenocrystsare rare (Table 3). The sieved Plg phenocrysts are xenocrystic andwere derived from the local basement (Meriggi et al., 2008). Thegroundmass mineral assemblage of Cerro del Agua is similar to that ofPelagatos with abundant labradorite laths (33.6–55.4 vol.%, Table 3),Ol, Cpx, Opx microcrysts, and glass.
The phenocryst content is markedly low in Dos Cerros rocks, andOpx and Plg phenocrysts are relatively abundant, in comparison toproducts from the other two volcanoes (Table 3). The predominatlysubhedral Ol and Cpx phenocrysts are smaller than those in Cerro delAgua and Pelagatos samples (b1 mm) and commonly display reactionbays and skeletal textures that indicate disequilibrium stages. Theyalso exhibit chrome-spinel inclusions. Some of the subhedral toeuhedral, prismatic, ~0.5 mm long Opx phenocrysts display skeletaltextures and/or Cpx cores. Plg phenocrysts are b1 mmwide and showdissolution textures. All phenocrysts occur as individual crystals or inrare clusters of Opx and Cpx with minor Plg and Ol that are up to1.5 mm in diameter.
The groundmass of Dos Cerros samples is similar to that ofPelagatos and Cerro del Agua samples. However, their Plg-laths areless anorthitic and some exhibit skeletal textures.
In conclusion, mineral assemblages in rocks from the threevolcanoes are similar. The mafic mineral compositions of Pelagatos,Cerro del Agua, and Dos Cerros rocks and their disequilibrium texturespoint to magmas that underwent rapid crystallization, minor en routefractionation, and thus had short times of residence in the crust. Theslightly distinct phenocryst assemblage of Dos Cerros rocks mighthowever point to some small degree of crystal fractionation. Asexpected, Ol contents decrease from the less evolved Pelagatos to the
most evolved Dos Cerros rocks. Finally, the occurrence of scarcexenocrystic quartz, as also described by Blatter and Carmichael (1998)at other MVB monogenetic volcanoes near Zitácuaro, is an unequiv-ocal indicator for the assimilation of at least small amounts of crustalmaterial.
5. Geochemistry
5.1. Analytical methods and sampling procedures
Twenty-eight samples were analyzed for major and trace elementcompositions at Activation Laboratories LTD, Ancaster, Canada.Results and analytical methods (including detection limits andanalytical uncertainties) are listed in Tables 4A, 4B and 4C. Thesamples were collected from proximal, medial, and distal lavas as wellas from dense bombs in scoria deposits in order to determinecompositional variability between earliest and latest stages of theeruptions.
Sr–Nd–Pb-isotope ratios were determined at the LaboratorioUniversitario de Geoquímica Isotópica (LUGIS), Universidad NacionalAutónoma de México, México DF, with a FINNIGANMAT 262 thermal-ionization mass spectometer in static mode. Results are shown inTable 5. Eleven samples (3 from Pelagatos, 4 from Cerro del Agua, 4from Dos Cerros) were analyzed in order to infer the origin of magmasthat produced the volcanic rocks.
5.2. Results
5.2.1. Major and trace element compositionsWhole rock silica contents of Pelgatos, Cerro del Agua, and Dos
Cerros samples range from 52.5 to 59.5 wt.% (Fig. 7). Rocks wereclassified based on SiO2 and total-alkali (Na2O+K2O) contents (Le Baset al., 1986) (Fig. 7). Pelagatos rocks plot in the basaltic andesitefield, whereas Cerro del Agua and Dos Cerros rocks include basalticandesite and andesite compositions. All rocks fall within the subalka-line field (Fig. 7) and follow a typical calc-alkaline trend when plottedon an AFM diagram (not included). CIPW-norm calculations of theanalyzed rocks indicate that Pelagatos rocks are Ol-Hy-normative,Cerro del Agua rocks are Hy-normative, and Dos Cerros rocks are Hy-to-slightly Qz-normative. Norms and Fe2O3/FeO adjustments werecalculated with the aid of a program of the Union College Schenectady,NY (www.union.edu/PUBLIC/GEODEPT/COURSE/petrology/norms.htm).
Table 3Modal mineralogical analysis (vol.%) of selected Pelagatos (Pel.), Cerro del Agua (C.A.), and Dos Cerros (D.C.) samples (over 800 points counted per sample). Phenocrysts arecrystalsN0.25 mm. Abbreviations: A, andesite; BA, basaltic andesite; P, proximal; M, medial; D, distal. Sampling locations are indicated in Fig. 3 and geodetic positions (latitude/longitude/altitude) are listed in Tables 4A, 4B and 4C.
SiO2 variations are small in Pelagatos and Cerro del Agua rocks, butlarger in Dos Cerros rocks (Tables 4A, 4B and 4C, Fig. 7). The variationsin Na2O+K2O contents are not remarkable for the entire suite of rocks(Fig. 7).
Major and trace element results were plotted in variation diagrams(Fig. 8), chondrite-normalized rare earth element (REE) diagrams(Fig. 9a–c), and primitive mantle-normalized trace element diagrams(Fig. 9d–f).
Cerro del Agua proximal and medial samples have slightly lowersilica contents than distal ones, while Dos Cerros proximal lavas andbombs have lower silica contents than the medial and distal lavasamples. Pelagatos samples show no clear variations of SiO2 withdistance from the vent.Moderate to slight negative correlations ofMgO,
CaO, and TiO2 (Fig. 8a,b)with increasing SiO2 are observed in Dos Cerrosand Pelagatos, but are not evident in Cerro del Agua samples. FeOTOT
shows a negative correlation with SiO2 in all samples (Fig. 8c). MgOcontents in Pelagatos (~10 wt.%) and Cerro del Agua (~7 wt.%) are veryhigh (Fig. 8a). This is rather unusual considering that these rocks are notbasalts and have relatively high SiO2 contents (Fig. 7). In consequence,the Mg# of Pelagatos is also high (~72) (Table 4A). Since this value isN68 and the MgO content is N8%, and Ni (~250 ppm) (Fig. 8e) and Cr(~400 ppm) contents are high, the Pelagatos magma fulfills therequirements for being considered primitive (Wallace and Carmichael,1999). On the other hand, TiO2-values are higher in the more evolvedDos Cerros rocks (Fig. 8g), but do not fall within the high-TiO2 rock-group as defined by Meriggi et al. (2008).
Table 4AWhole rock major and trace element analyses of Pelagatos samples. Analytical uncertainties are at the detection limits±100%, at 20 times the detection limits ±15–20%, and at 100times the detection limits, better than±5%. Samples were pulverized with mild steel devices which contaminate with Fe (0.1%). Abbrevations: INAA: Instumental NeutronActivation Analysis. FUS-ICP: Fusion-Inductively Coupled Plasma. TD-ICP: Total Digestion-Inductively Coupled Plasma. PPXRF: Pressed Pellet X-Ray Fluorescence. MULT INAA-TD-ICPis a combination of the INAA and TD-ICP analytical methods. #Mg=(wt.% MgO/40.31)/[(wt.% MgO/40.31)+0.85(wt.% FeOTOT/71.84) after Frey et al. (1978) and whereFeOTOT=total recalculated Fe as FeO (=0.8998×Fe2O3(T) after Irvine and Baragar (1971). asl=above sea level. Sc. bomb=scoriaceous bomb. Note: further details of analyticalprocedures are described in the Actlabs website at www.actlabs.com.
Sample type Sc. bomb Lava Lava Lava Lava
Sample number 0510 0516 0517 0518 0515
Latitude 19°05′34.60″ 19°07′23.42″ 19°06′47.77″ 19°06′29.08″ 19°05′59.41″
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Pelagatos contents delineate a flat REE pattern compared to Cerrodel Agua and Dos Cerros, and its REE abundances are lower (Fig. 9a).Dos Cerros shows a broader range of REE abundances (Fig. 9c) and isstrongly enriched in the light rare earth elements (LREE), and onlyslightly enriched in the heavy rare earth elements (HREE), generatinga steeper pattern. In fact, in Dos Cerros rocks, both the LREE (e.g. La,Fig. 8f) and the HREE (e.g. Ce, Table 4C) abundances are among thehighest in the entire SCVF. An Eu anomaly is absent in all patterns,indicating that plagioclase fractionation did not occur.
Pelagatos and Cerro del Agua show similar patterns in traceelement variation diagrams (Fig. 9d,e), but Cerro del Agua rocks aremore enriched in Pb. However, Guilbaud et al. (2009) report higher Pband other heavy-metal element contents for some of the Pelagatosbomb samples. The Dos Cerros trace element pattern exhibits a sub-parallel shift toward higher values in comparison to Pelagatos andCerro del Agua. The pattern is also steeper on the least incompatibleelements side to the right of the diagram (Fig. 9f). All three data setsshow small Nb, Ce, La, Th, Rb, and Ti troughs (Fig. 9a–f). In contrast,
they display variable peaks of K, Ba, Pb, Sr, and U. Unlike typical arclavas, LILE/HFSE ratios are not high as shown in the element variationdiagrams and the Ba/Zr diagram (Fig. 8h). These ratios are also amongthe lowest in the entire SCVF. Abundances of Nb (a highly-incompatible immobile element) are relatively high in all three rocksets. Dos Cerros rocks are more enriched in incompatible elementslike Zr (Fig. 8g), Ba, and K (K2O in Fig. 8d).
5.2.2. Sr–Nd–Pb compositionsSr–Nd–Pb-isotope ratios of selected samples analyzed in this study
are listed in Table 5 and plotted in Figs. 10 and 11. Sr–Nd–Pb analysesof SCVF rocks are still scarce. Available 143Nd/144Nd ratios show valuesthat range from 0.51274 to 0.51298, while 87Sr/86Sr ratios range from0.70354 to 0.70456 (Meriggi et al., 2008).
Pelagatos, Cerro del Agua, and Dos Cerros rocks show variable 87Sr/86Sr ratios (0.703539–0.704722) and εNd values fall between+4.84 and+0.72 (Fig. 10). Sr–Nd isotopic ratios of samples aremostly confined tothe mantle array. Dos Cerros samples are anomalous and tend towards
Table 4BWhole rock major and trace element analyses of Cerro del Agua samples (see legend of Table 4A).
Sample type Lava Lava Bomb Lava Lava Lava Lava Lava
Sample number 0511 0521 0520 0522 0512 0523 0530 0524
Latitude 19°04′58.14″ 19°05′24.9″ 19°05′21.4″ 19°03′13.5″ 19°04′40.87″ 19°01′34.3″ 19°00′34.2″ 19°01′22.9″
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the enriched mantle away from other fields, whereas Pelagatos andCerro del Agua aremore heterogeneous and plotwithin the typical SCVFrange (Fig. 10). Despite the otherwise primitive nature of Pelagatossamples, the relatively high 87Sr/86Sr values are noteworthy. Thesevalues cluster near 0.7041, except for the open diamond analysis ofSchaaf et al. (2005), which falls near the depleted mantle far from therest of the samples (Fig. 10). Two proximal samples fromCerro del Aguahave much lower 87Sr/86Sr ratios than its two distal samples.
Pelagatos, Cerro del Agua, andDos Cerros show roughly linear trendson the Pb-isotope diagrams (Fig. 11). Pelagatos and Cerro del Aguasamples fall within the western MVB field (Fig. 11), are slightly moreradiogenic than Popocatépetl values, and tend towards the bulksediment composition (SP in Fig. 11). All samples plot above theNorthern Hemisphere Reference Line (NHRL). The two more Pb-radiogenic Cerro del Agua samples, which have lower Sr-isotope ratios,belong to proximal samples. Dos Cerros samples are themost radiogenicand exhibit a positive linear trend that falls near or within the Pacific
Ocean sediment and intraplate-basalt fields (IP-B) (Fig. 11). Some of thesamples that define the intraplate-basalt field are SCVF rocks (LaGatta,2003). Note that two of the SCVF samples (Hijo del Cuauhtzin andTeuhtli, Fig. 11) of Arana-Salinas (2004) also plot close to Dos Cerrossamples. FromFig. 11, it becomes clear that the SCVF rocks canbedividedinto two groups of distinct Pb-isotope compositions: 1) the less Pb-radiogenic rocks that fall within thewesternMVB field, and 2) themorePb-enriched rocks that fall outside thewesternMVB field. However, it isalso possible that SCVF Pb-isotope compositions follow a continuouslinear trend. More Pb-isotope data would clarify this question.
6. Interpretation of geochemical data and discussion
6.1. Fractionation processes and evolution of erupted magmas
Fractional crystallization processes in SCVF magmas were docu-mented in previous studies (Wallace and Carmichael, 1999; Arana-
Table 4CWhole rock major and trace element analyses of Dos Cerros samples (see legend of Table 4A).
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Salinas, 2004; Siebe et al., 2004b; Meriggi et al., 2008; Guilbaud et al.,2009). The small variations in silica contents in Pelagatos and Cerrodel Agua rocks (Fig. 8) do not allow the identification of distinctfractional crystallization processes during the ascent of their parentalmagmas to the surface. However, even primitive magmas such asPelagatos with MgO contents between 8 and 10 wt.% do not representprimarymagmas and have undergoneminor Ol-fractionation (Wallaceand Carmichael, 1999). Unlike Cerro del Agua rocks, Dos Cerrossamples show correlations of MgO and CaO with respect toincreasing SiO2 (Fig. 8a,b) which indicates that the dominantfractionating phases were Ol and Cpx. Although less clear, Cerrodel Agua magmas may have also fractionated Ol and Cpx. Thenegative correlation of FeOTOT with increasing SiO2 (Fig. 8b) in rocksfrom the three volcanoes attests to the fractionation of FeTi-oxidephases. Nonetheless, the presence of chrome Sp-inclusions in Ol istypical for relatively primitive magmas that have not undergone asignificant fractionation of Cpx (e.g. Roeder, 1994) and itsoccurrence in magmas that reached the surface indicates thatfractionation of Ol and Sp was incomplete at depth (Schaaf et al.,2005). Since this petrographic characteristic is observed in the threerock suites (even in the most evolved samples from Dos Cerros),their parental magmas were probably modified only slightly byfractional crystallization processes.
fields representmagmas that have ascended rapidly to the surfacewithpossibly fewshort periods of stagnation (Siebe et al., 2004b, Blatter et al.,2007). Pelagatos magnesian Ol phenocrysts (Fo87–91 rich cores;Guilbaud et al., 2009) and their chrome Sp-inclusions are consistentwith a peridotitic mantle source (Blatter et al., 2007). Besides, Pelagatosmagmas entirely fulfill the criteria for a primitive magma (Fig. 8a,e,Table 4A) as defined by Wallace and Carmichael (1999). Thus, thenature of Pelagatos magmas would preclude extensive crystal fraction-ation. In addition, Cerro del Agua (#Mg=63–73, MgO=6–8 wt.%,Ni=140–208 ppm) (Fig 8, Table 4B) can be considered close to aprimitive composition. This is not the case of Dos Cerros rockswhich aremore evolved (#Mg=55–64, MgO=4–6 wt.%, Ni=29–110). They aremore silica-richas a result of crystal fractionation andOl is less abundant(Table 3). This fractionation must have occurred during rapid ascent(withoutmajor periods of stagnation at higher crustal levels), since low-pressure crystallization mineral assemblages typical of more evolvedandesitic–dacitic compositions are absent (e.g. Schaaf et al, 2005). Theporphyritic-poor composition of Dos Cerros rocks (phenocrysts b2%)(Table 3) also supports this interpretation since andesitic compositionsassociated with long residence times in the crust are typically highlyporphyritic (N20 vol.% phenocrysts, e.g. Schaaf et al, 2005).Near-aphiric,high-K/H2O andesites near Ollagüe volcano in the central AndeanCordillera were also interpreted to be the result of a rapid ascentthrough a 70-km-thick crust (Mattiolli et al., 2006). The presence ofskeletal Ol microphenocrysts and the absence of phenocrysts and/orgroundmass hornblende characterize these rocks, a feature that theyshare with Dos Cerros rocks. However, the presence of larger Olphenocrysts (plus Cpx-Opx-Plg phenocrysts) and major elementdistribution patterns in Dos Cerros rocks (Fig. 8) suggest a slowermagma ascent history (allowing for small degrees of crystal fraction-ation) than in the case of rocks near Ollagüe in the Andean Cordillera.
6.2. Genetic relationship between magmas
Pelagatos magmas have one of the most primitive compositionsknown in the SCVF. Hence, it could be argued that the evolved SCVFmagmas might have been derived from magmas similar to thosethat erupted at Pelagatos. However, several lines of evidence do notsupport this assumption. First, the plots in Fig. 8 show either distinctclusters of data-points or lines with different slopes. Second, REEand trace element patterns (Fig. 9) of Pelagatos and Cerro del Aguaare similar, but Cerro del Agua is slightly enriched in the REE, HFSE,and LILE. Although Dos Cerros REE and trace element patterns areparallel to the patterns of the other two volcanoes, they clearlyexhibit an enrichment in all elements. It has been noticed before inSCVF rock suites that the HFSE and the HREE become depleted withincreasing SiO2 (Márquez et al., 1999b; Verma, 1999; Siebe et al.,
Table 5Sr–Nd–Pb isotope ratios of Pelagatos, Cerro del Agua, and Dos Cerros selected samples. 60 isotopic ratios were performed for Sr and Nd, and 100 for Pb. Sr and Nd isotopic values wereadjusted to 87Sr/86Sr=0.710237±23 (±1σabs, n=355) for the NBS 987 standard and 143Nd/144Nd=0.511871±23 (±1σabs, n=178) for the La Jolla standard respectively. Resultswere corrected for mass fractionation by normalizing to 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219. The fractionation factor for Pb isotopic ratios was determined by comparisonwith the mean value of the Pb NBS standard (206Pb/204Pb=16.90±0.05%, 207Pb/204Pb=15.43±0.08% and 208Pb/204Pb=36.52±0.10% (±1σrel, n=164). 1sd and 1σ are thestandard deviations of the runs and are referred to the last two digits. Analytical blanks during the runs of these samples were 6.8 ng for Sr, 1.5 ng for Nd (total blank), and 46 pg forPb (chemical blank). Note: analytical procedures are described in Schaaf et al. (2005) and the LUGIS website (www.geologia.igeolcu.unam.mx/Lugis/manual.htlm).
Fig. 7. Total alkalis (Na2O+K2O) plotted against silica (SiO2) after Le Bas et al. (1986)for Pelagatos, Cerro del Agua, and Dos Cerros analysed rocks. The fields shown forreference are as follows: SCVF (selected samples from the western, central, and easternsectors of the SCVF taken from Siebe et al., 2004b, Arana-Salinas, 2004; Martínez-Serrano et al., 2004; Schaaf et al., 2005; Rodríguez-Huitrón, personal communication)(analyses chosen for each volcano include those with the highest and the lowest silicacontents); Popocatépetl after Schaaf et al. (2005); and Nevado de Toluca afterMartínez-Serrano et al. (2004). The line separating the alkaline from the subalkalinefield (MacDonald and Katsura, 1964) is also shown.
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2004b). Siebe et al. (2004b) associate this trend with thefractionation of HFSE and HREE-bearing phases. Since the fraction-ation of Cpx and Fe–Ti-oxides has been invoked in the evolution ofCerro del Agua and Dos Cerros magmas, these more evolved rocksshould display lower HFSE and HREE abundances in comparison toPelagatos primitive magmas. But this is not the case as shown by theprogressive enrichment of the HFSE and HREE from primitivePelagatos to more evolved Dos Cerros rocks (e.g. TiO2 and Zr,Fig. 8g). Third, at similar SiO2 contents, Dos Cerros and Cerro delAgua rocks show sub-parallel REE and trace element patterns(Fig. 9), but Dos Cerros have more enriched compositions. Inconclusion, the different magma batches evolved independentlyfrom each other during their ascent through the crust.
6.3. High-SiO2 primitive (MgO-rich) magmas
The presence of calc-alkaline primitive magmas in the SCVF hasbeen reported previously (e.g.Wallace and Carmichael, 1999; LaGatta,2003; Siebe et al., 2004b; Gómez-Tuena et al., 2007a; Meriggi et al.,2008; Straub et al., 2008; Guilbaud et al., 2009). The high SiO2 content(N50 wt.%) of many calc-alkaline primitive rocks in the SCVF (e.g.Wallace and Carmichael, 1999; Verma, 2000; Siebe et al., 2004b; Schaafet al., 2005) is noteworthy as primitive characteristics are typically
associated with basaltic (SiO2b50 wt.%) compositions (Wilson, 1989).Although Pelagatos and Cerro del Agua rocks are chemically classified asbasaltic andesites, petrographically they resemble basalts. Even themost evolved andesite rocks of Dos Cerros contain basaltic mineralassemblages. High-SiO2 primitive magmas in the SCVF have beenexplained in terms of crystal fractionation (Wallace and Carmichael,1999; Siebe et al., 2004b). However, the high-MgOPelagatos rocks showvery little evidence for Ol fractionation. Despite this, they have a highsilica content. This observation suggests that basaltic andesite Pelagatosand Cerro del Agua rocks (and to some extent also Dos Cerros rocks)could representprimary or near-primarymelts fromaperidotitic sourceunderneath central Mexico as postulated by Blatter and Carmichael(2001) based on phase-equilibria experiments. More recently, Straubet al. (2008) documented the occurrence of basaltic andesite andandesite rocks withMgO contents between 2 and 10 wt.% containing Olphenocrysts with moderate to high Ni contents in the central MVB, andproposed instead a hybridized peridotite/pyroxenite source thatpromotes the production of high-SiO2 primary calc-alkaline basalticandesite and andesite compositions. Interestingly, theNi-rich coresofOl(NiO=0.141–0.492 wt.%; Guilbaud et al., 2009) and high whole-rockMgO contents (~10 wt.%) in Pelagatos rocks suggest that their originmaybe related to thepartialmeltingof a peridotite/pyroxenite sourceasdescribed by Straub et al. (2008).
Fig. 8. Major (wt.%) and trace (ppm) element variation diagrams. For comparison, the field occupied by previously published SCVF analyses (see references in Fig. 7) is indicated.Popocatépetl (Schaaf et al., 2005) and Nevado de Toluca samples (Martínez-Serrano et al., 2004) were also plotted. Pelagatos samples analysed by Guilbaud et al. (2009) were plottedalong with the five Pelagatos samples collected during this study.
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Another process that could yield primitive silica-rich magmas isthe partial melting of a peridotititic source affected by H2O–SiO2-richmelts generated directly from the subducting slab (Gómez-Tuenaet al., 2007b). Geodynamical models (Johnson et al., 2009) indicatethat temperatures of the subducted oceanic crust are too low to allowpartial melting, which would rule out the slab-melt hypothesis.
Although no consensus about the exact origin of primitive or near-primitive high-SiO2 calc-alkaline rocks in the central MVB has beenreached, most studies (e.g. Wallace and Carmichael, 1999; Blatter andCarmichael, 2001; LaGatta, 2003; Siebe et al., 2004b; Blatter et al.,2007; Gómez-Tuena et al., 2007b; Meriggi et al., 2008; Straub et al.,2008; Guilbaud et al., 2009; Johnson et al., 2009) conclude in that
these melts were generated by the interaction of subarc-mantle rockswith hydrous components from the subducting slab. High LILE/HFSEand LREE/HFSE ratios and other geochemical criteria have been usedas sufficient evidence for hydrous slab-contributions. In addition,Johnson et al. (2009) proposed the production of sediment-meltsbeneath the Michoacán–Guanajuato Volcanic Field (MGVF), based on2-D thermodynamical models. Accordingly, the melts produced in themantle wedge should be enriched in the LILE and the LREE, acharacteristic feature of arc lavas (Gill, 1981; Wilson, 1989). But theLILE and the LREE contents in many SCVF rocks are not typical of arclavas (Luhr, 1997; Ferrari et al., 2000; Verma, 2000; Cervantes andWallace, 2003; Gómez-Tuena et al., 2003, 2007a; Siebe et al., 2004b;Meriggi et al., 2008; Straub et al., 2008), including Pelagatos, Cerro delAgua, and Dos Cerros rocks, all of which havemoderate LILE/HFSE andLREE/HFSE ratios (Figs. 8g and 9d–f).
6.4. Alkaline magmas and heterogeneous mantle
Peculiar alkaline primitive rocks, known as OIB (ocean islandbasalt) or intra-plate alkaline rocks, also occur in the central MVB(Wallace and Carmichael, 1999; LaGatta, 2003; Siebe et al., 2004b;Blatter et al., 2007; Straub et al., 2008; Johnson et al., 2009). Theserocks are enriched in the HFSE. For example, Nb abundances in SCVFalkaline rocks are N20–34 ppm and TiO2-contents are N1–2 wt.%(Cervantes andWallace, 2003; Siebe et al., 2004b;Meriggi et al., 2008;Straub et al., 2008). The origin of such rocks is under discussion andseveral scenarios (or their combinations) have been proposed,including: 1) an enriched mantle peridotite that moved by advectionunderneath the SCVF from deeper back-arck mantle regions by slab-induced corner flow (Luhr, 1997; Wallace and Carmichael, 1999;Siebe et al., 2004b); 2) a mantle plume (Márquez et al., 1999a);3) continental rifting (Verma, 2000); 4) roll-back of the subductedplatewith slab-windowopening(Ferrari et al., 2001); 5)decompression
Fig. 9. Graphs a, b, and c show chondrite-normalized REE compositions (ppm) and graphs d, e, and f show primitive mantle-normalized trace element abundances (Sun andMcDonough, 1989) in Pelagatos, Cerro del Agua, and Dos Cerros rocks (dark grey shaded areas). The light grey shaded areas represent the group of rocks from the entire SCVF (seereferences in Fig. 7). For comparison, Popocatépetl (Schaaf et al., 2005), Nevado de Toluca (Martínez-Serrano et al., 2004), Guespalapa, Chichinautzin, and Xitle (Siebe et al., 2004b)samples are also plotted.
Fig. 10. 87Sr/86Sr vs. ε-Nd diagram for Pelagatos, Cerro del Agua, and Dos Cerros samples(modified from Siebe et al., 2004b). Two additional Pelagatos analyses were included:one published by Meriggi et al., 2008 (black filled diamond), and another by Schaafet al., 2005 (open diamond). Popocatépetl and Nevado de Toluca analyses from Schaafet al. (2005), mantle array after DePaolo and Wasserburg (1979). EM=enrichedmantle; DM=depleted mantle.
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melting (Verma, 2000; Cervantes and Wallace, 2003; Siebe et al.,2004b); 6) partial melting of an amphibole-bearing mantle (Meriggiet al., 2008); and 7) serial melting of peridotite/pyroxenite mantlesources (Straub et al., 2008). Most hypotheses are reconciled in theidea of a heterogeneous mantle composition (Luhr, 1997; Wallaceand Carmichael, 1999; Cervantes and Wallace, 2003; Siebe et al.,2004b; Meriggi et al., 2008; Straub et al., 2008; Johnson et al., 2009).Despite the overwhelming evidence (see above) of the role ofthe subducted oceanic crust in the generation of the centralMVB calc-alkaline rocks, the alkaline rocks do not show typicalsubduction-related features (e.g. pronounced Nb anomaly, highLILE/HFSE ratios, etc.). For this reason, some authors have proposeda different tectonic setting, other than the subduction regime, forthe genesis of the alkaline and calc-alkaline rocks in the centralMVB (Márquez et al., 1999a; Verma, 2000; Márquez and De Ignacio,2002).
Pelagatos rocks do not show an enriched-HFSE composition.However, Cerro del Agua rocks display relatively high abundances ofNb (11–27 ppm) and Dos Cerros exhibit high contents of Nb (16–24 ppm), TiO2 (1.1–1.2 wt.%), and K2O (1.4–1.9 wt.%). AlthoughCerro del Agua and Dos Cerros rocks show moderate subduction-related characteristics, e.g. negative Nb anomalies and positive K–Ba–U anomalies (Fig. 9e,f), they can be considered as mildly alkalineand derived from a more enriched source than that of Pelagatos. Theclose relation in time and space of calc-alkaline and alkaline rocks inthe SCVF is of special interest and still an unresolved puzzle. Othercentral MVB rocks from the Zitácuaro–Valle de Bravo region (ZVB)(Blatter et al., 2007) and the MGVF (Johnson et al., 2009) showcompositional and spatial distributions that change systematicallyfrom the arc-front to the back-arc regions. In both cases, the back-arc rocks (located at N100 km from the arc-front) are more alkalineand have been associated with decompression melting of low-H2O(b1.5 wt.%) fertile asthenospheric mantle with little to no involve-ment of slab-derived fluids. Cervantes and Wallace (2003) suggest asimilar scenario for the production of Xitle lavas in the SCVF: lowwater contents (b1.3 wt.%) due to the lack of slab-fluids anddecompression melting. The chemical and petrographic character-istics of Dos Cerros rocks are similar to those of back-arc alkalinerocks. In addition, the higher abundance of Plg phenocrysts inDos Cerros rocks could indicate the presence of lower concentrations
of magmatic water. On the other hand, the arc-front rocks aredistinctly more water-enriched: 3.5–6.5 wt.% in the case of the ZVB(Blatter et al., 2007) and 3.0–5.8 wt.% for the MGVF (Johnson et al.,2009). Some of the rocks in the SCVF reported by Cervantes andWallace (2003) have water contents that are within these ranges.Pelagatos rocks water contents were estimated at b5 wt.% (Guilbaudet al., 2009) and resemble the chemical characteristics of themedium-K2O basaltic andesites of the ZVB. It appears that Pelagatosmagmas were more water-rich than Dos Cerros magmas. Thus, twovolcanoes that are only ~5 km apart (Dos Cerros and Pelagatos)(Fig. 3) and erupted in a time span of only a few thousand yearsdisplay quite different chemical, eruptive, and morphologic char-acteristics. This is not a unique case in the entire SCVF (e.g. Siebe et al.,2004b).
The subducting slab dips at ~30° to a depth of ~80 km underneaththe MGVF (Johnson et al., 2009) and at ~22° to a depth of ~70 kmunder the ZVB (Blatter et al., 2007). Based on seismic tomography,Pérez-Campos et al. (2008) estimated a vertical distance of 150 kmfrom the Earth's surface to the dipping subducting slab underneaththe Mexico City area where the Cocos Plate plunges with a dip of ~75°abruptly into the mantle after having a flat subduction. Therefore, it ispossible that the steep-dipping slab beneath the SCVF has implica-tions for the distribution in time and space of volcanism in this smallarea.
Straub et al. (2008) proposed that the serial melting of pyroxenite/peridotite mantle compositions and their hybrids is able to yield thediversity of lavas with different geochemical signatures that charac-terize the SCVF. Although most authors agree that HFSE-variations inthe central MVB rocks are large enough that they cannot be explainedby variations in the degree of partial melting of a single mantle source(Johnson et al., 2009), the above argument denies the existence ofmantle heterogeneities as postulated by Siebe et al. (2004b),restricting the mantle beneath the SCVF to a single moderatelydepleted composition. Although the exact distribution and extent ofsource heterogeneities are complex to assess, the following scenarioshave been invoked to explain mantle heterogeneities underneath thecentral MVB: a) a variable depletion of the mantle wedge caused byearlier partial melting events (Wallace and Carmichael, 1999), b) asub-arc mantle that is selectively influenced by variable fluidcompositions from the slab (Cervantes and Wallace, 2003), and
Fig. 11. a) 207Pb/204Pb vs. 206Pb/204Pb and b) 208Pb/204Pb vs. 206Pb/204Pb diagrams for Pelagatos, Cerro del Agua, and Dos Cerros rocks (modified from Arana-Salinas, 2004). The fieldsshown for comparison are as follows: Pacific Ocean sediments (Church and Tatsumoto, 1975; Plank and Langmuir, 1988), western MVB calcalkaline rocks (Luhr et al., 1989; Luhr,1997), Popocatépetl rocks (Schaaf et al., 2005) (dotted line), and sample data for SCVF monogenetic volcanoes (Ocusacayo, Atocpan, Tlacotenco, Hijo del Cuauhtzin, and Teuhtli)from Arana-Salinas (2004). Additional data include the intraplate basalts (IP-B) (long-dashed line field) from both the SCVF (LaGatta, 2003) and the Palma Sola Volcanic Field(Gómez-Tuena et al., 2003); weighted bulk sediment composition from DSDP site 487 is from Gómez-Tuena et al. (2003) and is shown as a shaded square labeled with SP in Fig. 11.NHRL=Northern Hemisphere Reference Line after Hart (1984).
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c) advection of the asthenospheric mantle from the back-arc regions(e.g. Luhr, 1997).
6.5. Mantle source and crustal contributions
Sr–Nd–Pb-isotope ratios can be used to constrain the compositionof source regions of magmas (Rollinson, 1993). The variations in the87Sr/86Sr ratios of Pelagatos, Cerro del Agua, Dos Cerros, and otherSCVF rocks are large compared to the Sr-isotopic variations displayedby the neighboring Nevado de Toluca and Popocatépetl strato-volcanoes (Fig. 10). Siebe et al. (2004b) and Schaaf et al. (2005)pointed out that the 87Sr/86Sr variations of the strato-volcanoes(Fig. 10) can be explained by crustal contributions in long livedmagma chambers. The same interpretation has also been used toexplain the scattering of 87Sr/86Sr ratios of SCVF rocks (Siebe et al.,2004b). Indeed, the increase in 87Sr/86Sr ratios with increasing SiO2
may indicate crustal contamination (Wilson, 1989). Additionally,Elliot (2003) suggests that the 87Sr/86Sr increase could also be theconsequence of fluid contributions imprinted with 87Sr/86Sr signa-tures from the altered oceanic slab. In the case of monogeneticactivitiy, Hansen and Nielsen (1999) infer only subtle crustalcontamination in magmas with short residence times in the crust. Incontrast, Mattiolli et al. (2006) propose crustal contamination of asuperheated andesitic magma that ascends without stagnationthrough a ~70-km-thick crust (87Sr/86Sr=0.7068). In short, theassessment of crustal contamination processes is complex.
The positive correlation of 87Sr/86Sr ratios with respect to thedegree of rock evolution is used as an indicator of crustalcontamination. Using this criterion, crustal contributions have beenproposed in SCVF magmas by Meriggi et al. (2008). These authors usethe MgO wt.% as an index of evolution. Clearly, the MgO-rich,primitive Pelagatos magmas (Fig. 10) show higher 87Sr/86Sr ratiosthan the MgO-poorer Cerro del Agua rocks. This contradictoryobservation requires an alternative explanation.
Dos Cerros rocks show an affinity towards an enriched mantle(Fig. 10) as underscored by their high HFSE contents (Fig. 9). Theserocks are more silica-rich (likely as a result of AFC processes) but theirmagmas did probably not have a long residence time in the crust.Nevertheless, due to a 45–50-km crustal thickness underneath theSCVF (Urrutia-Fucugauchi and Flores-Ruíz, 1996), they might havebeen contaminated en route to the surface in a similar way asdescribed by Mattiolli et al. (2006). Therefore, Dos Cerros andesiticmagma (with higher 87Sr/86Sr ratios, Fig. 10) might have beenmodified by crystal fractionation and crustal contamination ofprimitive alkaline, Xitle-type magma (lower 87Sr/86Sr ratios, Fig 10)on its ascent through a thicker crust.
Most Pelagatos primitive samples cluster closely in Fig. 10.Although all samples display similar petrographic and chemicalcharacteristics, Pelagatos sample No. 93365 (Schaaf et al., 2005) isisotopically anomalous and plots near the depleted mantle, far fromthe other four analyses. Such a shift could be explained in terms ofcrustal contributions. In fact, the initial lava emplacement tempera-ture of N1200 °C (Guilbaud et al., 2009), the presence of quartzxenocrysts, and the ~50 km crustal thickness, all point towards asmall crustal contribution whose extent is difficult to assess.Nonetheless, even if considering the possibility of contamination,the offset value of Pelagatos reported by Schaaf et al. (2005) is at odds.
Regarding Cerro del Agua rocks, the two samples with the highest87Sr/86Sr ratios have also slightly higher silica contents (Fig. 10), whiletheir Pb-isotope ratios are lower (Fig. 11). In constrast, the sampleswith the lowest silica content have comparatively lower 87Sr/86Srratios and higher Pb-isotope ratios. Radiogenic Pb-signatures ofvolcanic rocks associated with subduction magmatism are attributedto sediment contributions (Wilson, 1989). However, due to theinverse correlation between Pb and Sr-isotope ratios, radiogenic Pb in
Cerro del Agua rocks appears to indicate small-scale heterogeneitiesat the source in addition to sediment contributions.
Radiogenic Pb Pelagatos ratios plot near the bulk sediment field(Fig. 11) indicating a sediment contribution from the subductingslab. Johnson et al. (2009) proposed the involment of a sediment-melt component in the generation of central MVB magmas. Thiscould explain the REE enrichment of some of the primitive rockssuch as Chichinautzin and Xitle (Fig. 9a–c). Cerro del Agua shows ascattering that trends towards the bulk sediment field, but asexplained above, might be rather the result of mantle heterogene-ities. Dos Cerros more radiogenic 206Pb/204Pb and 207Pb/204Pb ratiosfall within the intra-plate basalt field (Fig. 11). Although Dos Cerrosrocks may be affected by crustal contamination, slab-componentcontributions should be negligible (e.g. Pb contributions); thus it ispossible that the 206Pb/204Pb and 207Pb/204Pb ratios of alkaline rocks(including Dos Cerros rocks) represent the original mantle valuesunderneath these regions. The scarcity of Pb-isotope analyses ofalkaline and calc-alkaline rocks in the SCVF (and other central MVBvolcanic fields) impedes a conclusion in regard to the abovediscussion.
In summary, the distinct Sr–Nd–Pb-isotope and element composi-tions and the small ejected volumes of Pelagatos, Cerro del Agua, andDos Cerros volcanoes suggest that each eruption was characterized bysmall magma batches that originated in an heterogeneous mantle.This mantle source has been affected for a long time by variablecontributions of fluids and perhaps sediment-melts from thesubducting slab. Each magma batch had its own separate history ofevolution and ascent to the surface.
7. Conclusions
The eruptions of closely-spaced Pelagatos, Cerro del Agua, and DosCerros monogenetic volcanoes were characterized by effusive lavaemissions and Strombolian explosive activities. The Dos Cerroseruption likely was more energetic than that of Pelagatos as impliedfrom the textural characteristics of their scoria cone deposits and theexistence of ash fallout deposits at Dos Cerros. Moreover, the totalerupted volumes of Pelagatos (0.038 km3) and Dos Cerros (1.25 km3)show a strong contrast. Their morphologic characteristics proved tobe unreliable in regard to constraining their relative ages, but wereotherwise a helpful tool for reconstructing their eruption andemplacement processes.
The three volcanoes are the result of recent eruptions (≤14,000 yrBP) and their associated magmas underwent small degrees offractional crystallization on their ascent to the surface. Frompetrographic and geochemical observations, it appears that Pelagatos,Cerro del Agua, and Dos Cerros magmas did not suffer muchstagnation during ascent through the crust.
Pelagatos, Cerro del Agua, and Dos Cerros rocks have interme-diate compositions. High-Mg basaltic andesite Pelagatos rocksrepresent primitive calc-alkaline magmas despite their high-silicacontent, which was probably inherited from the mantle source.Cerro del Agua rocks are near-primitive transitional rocks with OIB-type affinities that also have relatively high SiO2 contents. DosCerros andesite rocks are more fractionated rocks, but stillunderwent only low degrees of fractionation. They are related tothe high-HFSE alkaline rocks with minor involvement of a slab-component. The slab contribution, which metasomatized the mantlewedge, is clearly evident in Pelagatos rocks, yet minor in Cerro delAgua and Dos Cerros. The concept of different mantle sources(enriched and depleted) beneath the SCVF is reinforced by theresults of this study.
Mantle heterogeneities beneath the SCVF are necessary to explainthe variation in major, trace, and isotopic data between the threevolcanoes, but small degrees of crustal contributions likely overprintoriginal mantle signatures as in the case of Pelagatos. However, a
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chemically and isotopically distinct source is envisaged for eachvolcano. The extent of crustal contamination is not large, butdiscernible, and most evident in Dos Cerros rocks.
The three closely-spaced contemporaneous monogenetic volca-noes display a relatively wide range in their petrogenetic, magmatic,and eruptive characteristics. This could be associated with thepeculiar subducting slab geometry beneath Mexico City that gen-erates complex and varied melting processes in combination with adistinct extensional structural regime in the continental crust.
Acknowledgments
Laboratory work and field studies were supported by grantsCONACYT-50677-F and DGAPA-UNAM-IN1010063 assigned to C.Siebe. We thank Peter Schaaf, Gabriela Solís, Teodoro Hernández,Juan Morales (LUGIS-UNAM) for assistance with isotope analyses.Aerial photographswere provided byMoisésMartínez (Fotogrametríay Servicios Profesionales; S.A.). Satellite images were provided byMichael Abrams (Jet Propulsion Laboratory). Thin sections were madewith the assistance of Diego Aparicio. Lilia Arana provided theradiometric age of sample TML-65 (Lab.# AA-50118). AlejandroRodríguez shared unpublished geochemical data. The first authorwants to thank the assistance in the field of Hugo Murcia, Lilia Arana,Katrin Sieron, Sergio Salinas, Lorenzo Meriggi, and Juan R. Cruz. Thispaper is part of the Master's Thesis of the first author developed withstipends from Posgrado en Ciencias de la Tierra (UNAM) andCONACYT. Reviews by José Luis Macías and Dawnika Blatter werehelpful for improving this paper.
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